Air that is being compressed in a compressor, for example a rotary compressor, is often exposed to oil used to cool and/or lubricate the compressor. As a result, the compressed air can contain a mist of oil particles making the air unsuitable for operation of certain power equipment. In the past, filter assemblies or separators have been employed to remove the oil mist from compressed air. Not only must the separator effectively remove the oil, but must be designed so that it has a minimum restruction to flow.
U.S. Pat. No. 3,505,794 discloses a single stage separator including an annular filter element that is composed of a plurality of longitudinal pleats. The pleated filter element is formed of a plurality of layers of fibrous material, including fibrous material impregnated with activated charcoal, and the entrained oil in the air is coalesced as larger sized droplets which flow downwardly through the element for collection in a lower sump.
U.S. Pat. Nos. 3,085,381, 4,050,237 and 4,233,042 are directed to air-oil separators employing a two stage filter assembly in which the first stage serves to coalesce the entrained oil while the second stage acts to strip any remaining oil from the air. As disposed in the forementioned patents, the coalescing stage is composed of one or more layers of unwoven fibrous material, such as fiber glass.
It has been found that in the coalescor stage of the oil-air separators, as used in the past, a portion of the oil remains in the coalescor bed, saturating it, and thereby restricting the air flow through the coalescor stage. Further, the captured solids in the coalescor stage together with the contained oil, decreases the efficiency of the coalescor. This is due to the fact that the fibers in the coalescor are thin with the spacing between the fibers being minimal. Thus, the captured oil droplets form pools and patches drawing the fibers together and resulting in lower efficiency and an increased pressure drop.